P406: ENHANCED SIGNIFICANCE OF FLT3-ITD RESIDUAL DISEASE DETECTION ON TREATMENT OUTCOME IN ACUTE MYELOID LEUKEMIA
Authors: T. Grob, M. Sanders, C. Vonk, F. Kavelaars, M. Rijken, D. Hanekamp, P. Gradowska, J. Cloos, Y. Fløisand, M. V. Marwijk Kooy, M. Manz, G. Ossenkoppele, L. Tick, V. Havelange, B. Löwenberg, M. Jongen-Lavrencic, P. Valk
Journal: HemaSphere (2022)

Background: The prognostic significance of FLT3-internal tandem duplications (FLT3-ITD) in acute myeloid leukemia (AML) in relation to the allelic mutational burden and other concurrent gene mutations at diagnosis remains subject of scientific controversy. Increasing evidence indicates that treatment outcome prediction can be improved by assessing the kinetics and depth of response during therapy by detection of minimal residual disease (MRD). Until now, FLT3-ITD MRD detection by RQ-PCR has been hampered by the variety in patient specific FLT3-ITD (i.e. sequence, position and length) and is currently not recommended because of potential instability of FLT3-ITD clones during relapse. However, systematic studies evaluating the applicability of FLT3-ITD MRD detection with next-generation sequencing (NGS) in AML are currently lacking. Aims: Here, we set out to compressively investigate the impact of NGS-based FLT3-ITD MRD detection on treatment outcome in a cohort of newly diagnosed patients with AML in the context of current prognostic factors at diagnosis and other MRD measurements during therapy, including mutant NPM1 and multiparameter flow cytometry (MFC). Methods: In 176 de novo FLT3-ITD AML patients who were treated in HOVON-SAKK multicenter prospective phase III clinical trials, NGS was performed at diagnosis and in complete remission (CR) following two cycles of standard induction chemotherapy. The NGS libraries were paired-end sequenced (2×221-bp) with custom primers on an Illumina MiSeq according to manufacturer’s recommendation (Illumina, San Diego, CA). We used our in-house data analysis pipeline for variant calling. The primary endpoints of the study were relapse and overall survival. The Cumulative Incidence of Relapse (CIR) was estimated with competing-risks regression analyses according to the method of Fine & Gray. The Cox proportional hazard model was used to calculate overall survival estimates. Results: NGS-based FLT3-ITD MRD was present in 43 of 176 (24%) AML patients with a median variant allele frequency of 0.01% (range 3.1x10-4% to 3.10%). Presence of FLT3-ITD MRD was associated with increased risk of relapse (4-year risk of relapse, 79% FLT3-ITD MRD vs. 34% no FLT3-ITD MRD; P<0.001) and inferior overall survival (4-year rate of overall survival, 27% FLT3-ITD MRD vs. 57% no FLT3-ITD MRD; P<0.001). In multivariate analysis, detection of FLT3-ITD MRD in CR confers independent prognostic significance for relapse (hazard ratio, 4.00; P<0.001) and overall survival (hazard ratio 2.78; P<0.001). Strikingly, FLT3-ITD MRD exceeds the prognostic value of most generally accepted clinical and molecular prognostic factors, including FLT3-ITD allelic ratio at diagnosis and MRD assessment by NGS-based mutant NPM1 detection or MFC. Image:Summary/Conclusion: NGS-based detection of FLT3-ITD MRD in CR identifies AML patients with a profound risk of relapse and death that outcompetes the significance of most accepted prognostic factors at diagnosis and during therapy, and furnishes support for FLT3-ITD as a clinically relevant biomarker for dynamic disease risk assessment in AML.